At very high tracks per inch (TPI) in disc drives, non-repeatable runout (NRRO) becomes a major issue to achieve high performance tracking. NRRO comes from many sources such as electronic noises, disc flutter, spindle motor, arm/suspension modes, media defects, external vibrations, etc. Some of the sources have relatively high energy at certain narrow frequency bands (e.g., disc flutter and actuator resonance modes). There can be components of NRRO in several frequency ranges. At some outside diameter (OD) tracks, the sum of the NRRO energy at these multiple frequency ranges can comprise a significant portion of total NRRO energy. This may result in a long settling time at the end of seeking, increased write unsafe number, and large track following PES.
There is a desire to reduce disc drive NRRO in one or more narrow frequency bands without incurring the cost of increasing runout at other frequencies.